Electromagnetic actuators contain a cylindrical coil of insulated wire called a solenoid (hereinafter referred to as a coil). Energization of a coil by a DC voltage source creates a steady current flow through the coil which produces an axial magnetic field. The direction of the magnetic field is dependent upon the direction in which the coil conductor is wound and the direction of the current flow through the coil.
Electromagnetic actuators also contain a movable ferromagnetic component called an armature. The armature is located within the magnetic field and is subjected to a force, created by the field, which is proportional to the strength of the field. This force causes the armature to move in the same direction as the magnetic field.
Deenergization of a coil (removing the DC voltage source) interrupts the current flow. As a result, the magnetic field collapses and the force dissipates. Typically, the armature is returned to its original position by means of a spring.
The time it takes an armature to complete its movement in the direction of the magnetic field upon energization of the coil and the time it takes the armature to return to its original position upon deenergization of the coil are hereinafter referred to as the armature opening and closing response times, respectively.
Electromagnetic actuators are used in numerous applications requiring rapid armature response times, particularly in the area of fuel injectors for internal combustion engines. Rapid opening and closing responses improve the linear flow range of the injector and enable more precise fuel delivery.
An armature's response is affected by several factors, most notably the strength of the magnetic field to which it is subjected. The stronger the field, the greater the force acting upon the armature and the faster the opening response. Since the strength of a magnetic field is proportional to the current flow through the coil, one possible approach for improving the opening response is to increase the current flow through the actuator coil. This would, however, adversely impact the closing response since a stronger field also takes longer to collapse upon deenergization. Maintaining the additional coil current would also increase the overall power consumption of the actuator.
The affects upon the closing response could be minimized by using a switching circuit to boost the coil current only until the armature has reached the open position. The additional magnetic field strength supplied by the increased coil current is not required to maintain the actuator in the open position. This would improve the opening response time by creating a stronger initial magnetic field. In addition, by eliminating the problems associated with a stronger field to collapse upon deenergization, the closing response would not be adversely affected. Unfortunately, a switching circuit is required to turn the current boost on and off at the appropriate times, adding to the expense and complexity of operating the actuator.
An alternative approach might include the addition of a second coil in the actuator. The second coil could be used to supplement the magnetic field of the main actuator coil during energization and thereby improve the opening response. Conversely, it could be configured to assist the breakdown of the residual magnetic field in the main actuator coil during deenergization and improve the closing response. In either case, a switching circuit is required.